Introduction
The Medusa Radiometrics RhoC5 measures field bulk density and soil moisture content directly in the field. Field bulk density is an important physical soil health indicator and is generally used for assessing soil quality. The RhoC5 measures the backscatter of gamma radiation emitted from a (low-activity) source. The sensor can measure up to 80 centimeters deep and only needs a hole of 30 mm diameter for such a measurement. With this sensor, there is no need for a soil pit or any laboratory work to determine soil bulk density. The sensor provides the measured densities during measurement in the field. This allows the soil scientist to gather more information on the vertical as well as the horizontal scale.
The housing is rugged, easily portable, and can be operated by any field worker.
The RhoC5 density meter is controlled over WiFi by an app on an android smartphone. The data acquisition is monitored by the smartphone and is stored locally on the phone. The user of the app can export the data from the smartphone to a PC, or save it in the cloud.
The RhoC5 contains three sensors: a density sensor, a depth sensor, and a soil moisture sensor. The density is determined over a layer of approximately 80 mm. The depth sensor uses radar technology and returns the depth of the center of the density measurement. The moisture sensor is located in the nylon tip of the RhoC5 and measures the soil moisture content over a layer of approximately 80 mm. The center of the moisture measurement is located 100 mm below that of the density measurement.
Working principle of the density sensor
The figure above demonstrates schematically how the density measurement works. The small red disk contains a radioactive 22Na source with a low activity below the exemption value. This source will radiate gamma-rays into the soil. This soil will interact with these gamma-rays. One of the interactions that can occur is known as Compton scattering, where the direction and energy of the gamma-ray is altered. The green cylinder in the figure is a scintillation crystal that can detect gamma-rays when they interact with the crystal. Between the crystal and the source, a piece of tungsten is positioned. Tungsten has a very high density, which prevents most radiation from the source from reaching the crystal directly. The black nylon part at the bottom contains the moisture sensor. As a result, most radiation registered by the scintillation crystal has been scattered in the soil surrounding the RhoC5. With increasing soil density, more radiation will be scattered, and more gamma-rays will be registered. The RhoC5 has been calibrated to determine the soil density based on the number of gamma-rays detected per second.
The radioactive source
The RhoC5 contains a radioactive 22Na source, which is constantly emitting gamma radiation and cannot be switched off. The activity of this source is below the exemption value, and no permit is required to operate and store the device.
Exempt means that the radioactive source is not subject to certain regulations, restrictions, or requirements that would normally apply to radioactive materials. The source is considered to have such a low level of radioactivity that it does not pose a significant health or safety risk, and therefore, does not require strict regulatory control.
Nevertheless, it is always important to keep exposure to any source to a minimum. The ALARA principle can be used for this, which stands for As Low As Reasonably Achievable. We advise to reduce the proximity of the source as much as possible. This means storing the device at a distance from people and using shielding, such as a wall, when possible. Try to reduce the time in proximity to the RhoC5. When preparing the next measurement, leave the RhoC5 pushed down into the previous borehole. The borehole will shield the radiation and prevent exposure.
Due to decay, the activity of the source will reduce over time. This is a physical property independent of whether the RhoC5 is being used. A lower source activity will result in a measurement taking longer to complete. After five years, the source will be so low that it should be replaced. Therefore, The RhoC5 sensor has to be shipped to the office of Medusa Radiometrics. Here, the 22Na source is removed from the device and the 22Na source(s) will be disposed off in accordance with the legal regulations. When the source is replaced, the sensor can be used for another five years.
More information on the radioactive source can be found on the Safety note page.